1. Field of the Invention
The invention relates to a control system for controlling distribution of power from a battery and an engine in a powertrain for a hybrid electric vehicle.
2. Background Art
A powertrain for a hybrid electric vehicle has two sources of power. One source typically includes an internal combustion engine. The second source of power includes a high voltage electric motor, usually an induction motor. One class of hybrid electric vehicle powertrains has parallel power flow paths.
The first power source for a parallel hybrid electric powertrain configuration comprises a combination of an engine, a motor, a generator, a high voltage battery and a planetary gearset for coordinating power distribution. The second power source comprises a combination of an electric drive system with a motor, a generator and a high voltage battery. The high voltage battery acts as an energy storing device for the generator and the motor. A low voltage battery is used to power the vehicle accessories, a powertrain controller and engine spark.
Engine output power for the first power source can be divided into two power flow paths by controlling the generator speed. A mechanical first power flow path is established from the engine to the planetary gear gearset, and ultimately to a transmission power output shaft. A second power flow path is an electro-mechanical power flow path that distributes power from the engine to the generator to the motor, and hence to the power output shaft of the transmission. The generator in this instance can be used to distribute torque to the gearset. With this powertrain configuration, the generator, the motor and the gearset may operate as an electro-mechanical transmission with continuously variable ratio characteristics.
By braking the generator with a selectively actuated brake, engine output power can be transmitted with a fixed gear ratio to the power output shaft of the transmission.
The electric motor of the second power source, may draw power from the high voltage battery and provide driving torque independently of the engine during both forward and reverse drive modes. Further, the generator can draw power from the high voltage battery and drive the vehicle forward using a one-way reaction brake on an engine power output shaft. In this instance, when the generator drive mode is effective, the generator acts as a motor.
The powertrain is under the control of a vehicle powertrain control module or controller, which coordinates the two sources of power to optimize total powertrain efficiency and performance while satisfying the driver's demand for power. The vehicle control system determines when and what each functioning component and subsystem should be used to satisfy the driver's demand for power. The function of the battery-motor-generator subsystem is lost, however, if the battery experiences an open circuit condition while the vehicle is in motion. The divided power delivery hybrid vehicle powertrain then cannot operate and the vehicle system controller will command the vehicle to shut down. This is due to the fact that the controller then cannot control distribution of current between the motor and the generator because the high voltage battery, which no longer is in the circuit, cannot perform its usual function as a capacitor to regulate bus voltage between the motor and the generator. The electrical connection between the generator and the motor then would be characterized by unstable, non-linear, voltage spikes.
Vehicle shut down under this abnormal powertrain condition is a result of the inability of the system to transmit power from the engine to the vehicle traction wheels through a mechanical power flow path, which requires either generator control or application of the generator brake. The generator cannot be controlled under this abnormal condition, however, because the high voltage battery is not in the circuit. Thus, the only way that power can be transmitted from the engine to the vehicle traction wheels through a mechanical torque flow path is to apply the generator brake. This results in a fixed gear ratio between the engine speed and the vehicle speed, as explained above, but it will cause the engine to stall when the engine speed falls below idle speed.